Need help with Aerocap for 05 Dodge Ram.

[Xist]

What percentage of gain for a 900% increase in effort?

[Hersbird]

Is it better because of cross wind? I don't think it looks tougher to make. Build it with thin foam and then cover it in epoxy on both sides.

[ChazInMT]

I can't get my head around how the TVargo Cap would be a dramatically better Cd. I still think a more squarish cap would have more usable space and be a bit simpler to build with common materials.

Something that doesn't get talked about as much as it should is "grand vortex formation" when ever you have a low pressure area next to a high pressure, you get the air wanting to move from high to low, and since you're hauling down the road, this is going to create a spinning motion of the air.

We must remember, the air is just sitting there (Ideally on a calm day) and we are ramming through it with our vehicles. The energy required to move the air out of the way, and then the energy we put into it to make it swirl about, is all felt as drag. The more the air is set in motion behind the vehicle, the more energy it takes to do that. So if you have a constant fairly high velocity swirl going on behind you as you drive, this requires energy to sustain. If the air behind you settles out quickly, and returns to its calm state in short order, than you have not put much energy into it and therefore you will feel less drag.

So it is these pressure differences in close proximity that we are looking to avoid in order minimize stirring up the calm air, and thus reduce drag.

Swirled ice cream cones GOOD, Swirled air BAD. My basic rule of thumb.

[t vago]

My aerocap is covered on the outside with epoxy. Inside is bare Styrofoam. I figure that the fuel logs show about a 12% increase from no cap. It's slightly better than the performance of this earlier aerocap.





And this is why I changed shapes.



The air will conform as much as possible to the sides of a vehicle as the vehicle moves through the air. Air that wants to move sideways, will continue to move sideways until there is no more surface to guide it. Similarly, downward moving air will move downward as long as there is a surface for the air to cling to. When the air no longer has anything to cling to, the various air streamlines will interact with each other. Vortices will form as air moving inward to the vehicle's center meets with air moving downward.

[freebeard]

If that were verified through tuft testing, then you wold have something you could really point to. Short tufts on the body and really long ones at the trailing edge.

It certainly looks plausible.

[t vago]

Originally Posted by freebeard

If that were verified through tuft testing, then you wold have something you could really point to. Short tufts on the body and really long ones at the trailing edge.

It certainly looks plausible.

Rain-induced dirt patterns verified the streamlines on the aerocap itself. As for the long tuft action, I actually saw these two vortices being generated in the rain. Didn't have a smartphone at the time. I suppose I could take a vid later on this year, because I still have the version 2 aerocap.

[freebeard]

One of the most instructive things to do in a wind tunnel is 'stuff the wake' by inserting the smoke wand behind the vehicle and letting it be pulled forward to the separation line.

It occurs to me that an Army smoke grenade would be perfect. aerohead has a story about setting one off on his surfboard. ...maybe a homemade variant with shorter duration?

[aerohead]

Originally Posted by Vajra

Okay, now it all makes sense. I have to wonder though, with the back end being like T Vago's, wouldn't that be less aerodynamic than just terminating at the tailgate? While I can see air moving and transitioning more smoothly along the top/sides, I would suspect that the tail end would not have much less drag verses a tail end that ended like a aerolid such as Bondos, or Swede's. I understand the concept of CL verses CD, but I have a hard time seeing how air flow being smoothed out would have a greater effect on efficiency. Or... Are we talking about having the back at 7-8" off the tailgate, with a similar curvature for the tail end? If I was do to what T Vago is doing, I would use foam as well, bread and butter style vertical sections. Don't think ply wood would be so easy to deal with in the larger lumber sizes. If 1/8" inch, sure... not 1/2" though. I think I would want slightly rounder edges and a flatter center though. My thinking, the edges on T Vago's tail end, would have a not so smooth arcing action of air on the sharper corners. Or am I mistaken? I am NOT knocking T Vagos work in progress, that cap looks very nice. I just don't see how it would be benefit reducing the CD as much as other options. I will never go out further than the tailgate down. As my truck is a 2500 4x4 with 4:10 gears, I don't see a possibility of much over 2mpg max with the aero cap. If I do, I think it would be to the point of losing much of the useful area of the bed. If I had to open it to carry cargo, what would the effect of the rather round end be if open say 1.5 feet? I suspect because of the shape, shallow corners, I'd end up having to open further up to accommodate a similar load. Hmmm... And yes... I want to hone in on a optimal shape, but also, most useful design possible. Construction wise, I think I can do just about anything one the design part is done.... at least I think so...

Streamlining is all about protecting the boundary layer in the aft-body such that no pressure rise is sufficient enough to trigger separation.In ballistics it's referred to as 'sectional density,' in aviation,'area rule.
The body cross-section cannot reduce more than a certain amount at any given location on the body,or the flow will stall,begin to back-flow,and roll up into eddies,then full-blown turbulence.
It all has to do with local air velocity/static pressure.
In a streamline body,the cross-section is identical for any given location,purely symmetrical .

That's not a 'practical' shape for cars,nor the 'half-body' created from sawing the body in half,lengthwise.
It's okay,to stray from the 'ideal' shape as long as we respect the velocity/pressure profile along the body.
Here is the Morelli shape,Cd 0.16,which is a compromise concession to 'practicality.'

At any point in the aft-body,Morelli is very careful to reduce the cross-section only gradually,to maintain attached flow.All the body cross-section are an ellipse.Those radius'd edges are a key to the low drag.
Aptera used all this curvature

Same with MIT

More 'conventional' cross-sections can still produce very low drag,as long as the 'soft' edges are maintained.Here's CUER

It has the same Cd 0.10 as HONDA's Dream-2 car

The soft edges allow air velocities and pressures to gain a more homogeneous value akin to the ideal streamline body.

[aerohead]

Originally Posted by freebeard

One of the most instructive things to do in a wind tunnel is 'stuff the wake' by inserting the smoke wand behind the vehicle and letting it be pulled forward to the separation line.

It occurs to me that an Army smoke grenade would be perfect. aerohead has a story about setting one off on his surfboard. ...maybe a homemade variant with shorter duration?

My brother knows a Hollywood special-effects guy who told him about 'smoke-cookies' that are used in films.They come rated in CFM of smoke,50,000 cfm for instance.
Parachute demonstration teams use something like the smoke grenades during public exhibitions.
'Guess we could check online 'n see if it's legal to purchase this stuff.
Toxicity would also be something to check on.

[Vajra]

Xist... After the government realizes your effort surpasses all known FE, they arrest you and take your truck. So... 900% effort ends in 99 years of solitary confinement No gains...

I have to agree with Chaz on the space and ease of construction. But here is something I am wondering about... What happens if you need to carry a load in your truck, which means the tail end of the cap is up in the air by 2-3'? I would think at this point, the flatter looking hatch would be more CD friendly, than what we are talking about. While area rule is good, I don't think we have the speeds or length enough to make it a huge factor. If the trailing area of the cap, under both circumstances keep the boundary layer of air attached, I would assume that the least amount of area at the end, would allow the greatest amount of drag reduction. At the same time, the question of usable space and the altering of the properties plays into my mind when I think of strapping down the aerocap for a normal size load. Last thing that would help would be a giant air brake!

With Morelli shape, my issue would be this... the very fact it is top to bottom a shape. I agree totally, very aero-sexy. It doesn't have any real sides. It is clean in shape. What we are adding, is a cap to make up for the bad CD, and try to lower it. His model, has a "flat" tail for the most part. If you were to use the shape shown, the tailgate end of the cap should remain as flattening out, no? It seems to me like we are comparing aerodynamics for car, rather than a semi. Yes both can be reduced, hugely. It comes at a cost though. You can make a highly aerodynamic semi.. the questions is, how much cargo space is there at that point, what is going to be the major trade off.. I think as far as practical means go, for design, building, and use, Chaz, Bondo, Swede are about spot on. Yes, there are most aerodynamic shapes, but to put into work, I think there would be some unforeseen issues.

I was going to show something between Chaz and my edit for the cap. But, I failed today. My editing skills are like my childhood... poor most of the time. Next attempt... maybe I will try a semi rigid inflatable aerocap! Best of all worlds.

I do truly appreciate all the thought and effort put into this thread. The ideas, thoughts. You guys are some of the smartest and nicest people I have seen on forums. I really enjoy the ideas being shared. Thank you, to all of you.